Methods and devices for posterior stabilization

ABSTRACT

Various methods and devices for replacing damaged, injured, diseased, or otherwise unhealthy posterior elements, such as the facet joints, the lamina, the posterior ligaments, and/or other features of a patient&#39;s spinal column, are provided. In certain exemplary embodiments, a four bar linkage mechanism can be used to construct spinal stabilization devices and methods for restoring function to adjacent vertebrae. In particular, spinal stabilization devices can be provided that kinematically form a four-bar linkage mechanism with adjacent vertebrae and a disc or other element disposed between the adjacent vertebrae.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 10/955,207 (Atty. Docket No. 101896-273), filed on Sep. 30, 2004 and entitled “Posterior Stabilization Systems and Methods,” U.S. patent application Ser. No. 10/905,374 (Atty. Docket No. 101896-296), filed on Dec. 30, 2004 and entitled “Artificial Facet Joints,” U.S. patent application Ser. No. 10/905,376 (Atty. Docket No. 101896-298), filed on Dec. 30, 2004 and entitled “Posterior Stabilization System,” and U.S. patent application Ser. No. 10/908,882 (Atty. Docket No. 101896-328), filed May 31, 2005 and entitled “Facet Joint Replacement.” These references are hereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

The vertebrae in a patient's spinal column are linked to one another by the disc and the facet joints, which control movement of the vertebrae relative to one another. Each vertebra has a pair of articulating surfaces located on the left side, and a pair of articulating surfaces located on the right side, and each pair includes a superior articular surface, which faces upward, and an inferior articular surface, which faces downward. Together the superior and inferior articular surfaces of adjacent vertebra form a facet joint. Facet joints are synovial joints, which means that each joint is surrounded by a capsule of connective tissue and produces a fluid to nourish and lubricate the joint. The joint surfaces are coated with cartilage allowing the joints to move or articulate relative to one another.

Diseased, degenerated, impaired, or otherwise painful facet joints and/or discs can require surgery to restore function to the three joint complex. Subsequent surgery may also be required after a laminectomy, as a laminectomy predisposes the patient to instability and may lead to post-laminectomy kyphosis (abnormal forward curvature of the spine), pain, and neurological dysfunction. Damaged, diseased levels in the spine were traditionally fused to one another. While such a technique may relieve pain, it effectively prevents motion between at least two vertebrae. As a result, additional stress may be applied to the adjoining levels, thereby potentially leading to further damage.

More recently, techniques have been developed to restore normal function to the facet joints. One such technique involves covering the facet joint with a cap to preserve the bony and articular structure. Capping techniques, however, are limited in use as they will not remove the source of the pain in osteoarthritic joints. Caps are also disadvantageous as they must be available in a variety of sizes and shapes to accommodate the wide variability in the anatomical morphology of the facets. Caps also have a tendency to loosen over time, potentially resulting in additional damage to the joint and/or the bone support structure containing the cap.

Other techniques for restoring the normal function to the posterior element involve arch replacement, in which superior and inferior prosthetic arches are implanted to extend across the vertebra. The arches may have rigid surfaces that can articulate relative to one another to replace the articulating function of the facet joints. However, aligning two articulating rigid surfaces for facet replacements can be very difficult given the variations in patient anatomy and various motion required (i.e., flexion, extension, lateral bending, and translations).

Accordingly, there remains a need for improved systems and methods for stabilizing adjacent vertebrae and more preferably for restoring normal function to adjacent vertebrae.

FIELD OF THE INVENTION

The present invention relates to methods and devices for stabilizing posterior elements of the spinal column.

BRIEF SUMMARY OF THE INVENTION

The present invention provides methods and devices for stabilizing adjacent vertebrae, and in particular exemplary methods and devices that utilize a four bar linkage concept are provided for restoring function to adjacent vertebrae. In one exemplary embodiment, an implant for stabilizing adjacent vertebrae is provided and it includes a body that is adapted to couple to first and second adjacent vertebrae and that is adapted to move relative to the adjacent vertebrae such that the body kinematically forms a four bar linkage mechanism in the sagittal plane with the adjacent vertebrae and a disc disposed between the adjacent vertebrae.

The body can have a variety of configurations to kinematically form a four bar linkage with the adjacent vertebrae and the disc between the vertebrae, but in one exemplary embodiment the body can include two joints that are adapted to allow movement of the body relative to adjacent vertebrae. The joints can be, for example, two sliding joints, or one sliding joint and one rotating joint. One exemplary sliding joint includes, by way of non-limiting example, a sleeve that is adapted to slidably receive a rod. One exemplary rotating joint includes, by way of non-limiting example, a ball that is adapted to be rotatably disposed within a socket. In use, the joints can allow flexion and extension of adjacent vertebrae.

In another embodiment, an implant for stabilizing adjacent vertebrae is provided and includes a rigid body having a first joint for movably coupling to a first vertebra and a second joint for movably coupling to a second adjacent vertebra. The first and second joints are each adapted to provide one degree of freedom in a sagittal plane when coupled to adjacent vertebrae such that the rigid body is adapted to restore function to adjacent vertebrae coupled thereto in combination with a disc disposed between the adjacent vertebrae. While the configuration of the joints can vary, in one embodiment a center of rotation of each of the first and second joints can be positioned substantially horizontal relative to one another. In another embodiment, at least one of the first and second joints can be adapted to slide vertically when the first and second joints are coupled to adjacent vertebrae.

In yet another exemplary embodiment, an implant is provided for stabilizing adjacent vertebrae in a patient's spine and the implant includes a first linkage that is adapted to rigidly couple to a first vertebra, a second linkage that is adapted to rigidly couple to a second adjacent vertebra, and a body for movably connecting the first and second linkages. When implanted, the first and second linkages and the body are adapted to kinematically form a four bar linkage mechanism in the sagittal plane with a disc disposed between the adjacent vertebrae to restore function to the adjacent vertebrae.

The body can have a variety of configurations, but in one embodiment the body can include first and second joints that are adapted to allow movement of the body relative to the first and second linkages. The first and second joints can each be adapted to slidably move relative to the first and second linkages, or alternatively one of the first and second joints can be adapted to slidably move relative to one of the first and second linkages, and the other one of the first and second joints can be adapted to rotatably move relative to the other one of the first and second linkages. Sliding movement can be achieved using, for example, a sleeve that is adapted to slidably receive a rod formed on the first and second linkages, and rotating movement can be achieved using, for example, a ball that is adapted to be rotatably disposed within a socket formed on one of the first and second linkages. The sleeve can, in certain exemplary embodiments, be formed in the ball.

In other aspects, a method for restoring function to adjacent superior and inferior vertebrae is provided and includes coupling adjacent superior and inferior vertebrae with a moving linkage to kinematically form a four bar linkage mechanism in the sagittal plane with the adjacent superior and inferior vertebrae and a disc disposed between the adjacent superior and inferior vertebrae, thereby restoring function to the adjacent superior and inferior vertebrae. The disc can be a natural disc or it can be an artificial disc that is adapted to allow movement between the adjacent superior and inferior vertebrae. Where an artificial disc is used, the method can include the step of implanting the artificial disc between the adjacent superior and inferior vertebrae.

Various techniques can be used to couple adjacent superior and inferior vertebrae with a moving linkage, but in one embodiment a first member can be rigidly mated to a superior vertebra, a second member can be rigidly mated to an inferior vertebra, and the moving linkage can be movably coupled to the first and second members. The moving linkage can be adapted to slide relative to both of the first and second members, or alternatively it can be adapted to slide relative to one of the first and second members and to pivot relative to the other one of the first and second members.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram illustration a four bar linkage mechanism for use in constructing a spinal stabilization device to restore function to adjacent vertebrae in a patient's spinal column;

FIG. 2A is a posterior view of one embodiment of a spinal stabilization device coupled to adjacent superior and inferior vertebrae;

FIG. 2B is a side view of the spinal stabilization device shown in FIG. 2A;

FIG. 2C is a side view showing the spinal stabilization device of FIG. 2B kinematically reduced to a form a four bar linkage mechanism in the sagittal plane;

FIG. 2D is a diagram illustrating the four bar linkage mechanism shown in FIG. 2C;

FIG. 2E is a diagram illustrating the four bar linkage mechanism shown in FIG. 2D, showing an infinite linkage;

FIG. 3A is a side, partially transparent view of another embodiment of a spinal stabilization device coupled to adjacent superior and inferior vertebrae;

FIG. 3B is a side view showing the spinal stabilization device of FIG. 3A kinematically reduced to a form a four bar linkage mechanism in the sagittal plane;

FIG. 3C is a diagram illustrating the four bar linkage mechanism shown in FIG. 3B;

FIG. 4A is a posterior view of yet another embodiment of a spinal stabilization device coupled to adjacent superior and inferior vertebrae;

FIG. 4B is a side view of the spinal stabilization device shown in FIG. 4A;

FIG. 4C is a side view showing the spinal stabilization device of FIG. 4B kinematically reduced to a form a four bar linkage mechanism in the sagittal plane;

FIG. 4D is a diagram illustrating the four bar linkage mechanism shown in FIG. 4C;

FIG. 5A is a side view of a disc implant disposed between adjacent superior and inferior vertebrae, showing the disc implant kinematically reduced to form a portion of a four bar linkage mechanism in the sagittal plane;

FIG. 5B is a diagram illustrating a portion of the four bar linkage mechanism shown in FIG. 4A;

FIG. 6A is a diagram illustrating one embodiment of construct for a spinal stabilization device having a sliding joint and a pivoting joint to kinematically form a four bar linkage mechanism;

FIG. 6B is a diagram illustrating another embodiment of construct for a spinal stabilization device having a sliding joint and a pivoting joint to kinematically form a four bar linkage mechanism;

FIG. 6C is a diagram illustrating another embodiment of construct for a spinal stabilization device having a sliding joint and a pivoting joint to kinematically form a four bar linkage mechanism;

FIG. 6D is a diagram illustrating another embodiment of construct for a spinal stabilization device having a sliding joint and a pivoting joint to kinematically form a four bar linkage mechanism;

FIG. 6E is a diagram illustrating another embodiment of construct for a spinal stabilization device having a sliding joint and a pivoting joint to kinematically form a four bar linkage mechanism;

FIG. 6F is a diagram illustrating another embodiment of construct for a spinal stabilization device having a sliding joint and a pivoting joint to kinematically form a four bar linkage mechanism;

FIG. 6G is a diagram illustrating another embodiment of construct for a spinal stabilization device having a sliding joint and a pivoting joint to kinematically form a four bar linkage mechanism;

FIG. 6H is a diagram illustrating another embodiment of construct for a spinal stabilization device having a sliding joint and a pivoting joint to kinematically form a four bar linkage mechanism;

FIG. 7A is a diagram illustrating one embodiment of construct for a spinal stabilization device having two pivoting joints to kinematically form a four bar linkage mechanism;

FIG. 7B is a diagram illustrating another embodiment of construct for a spinal stabilization device having two pivoting joints to kinematically form a four bar linkage mechanism;

FIG. 7C is a diagram illustrating another embodiment of construct for a spinal stabilization device having two pivoting joints to kinematically form a four bar linkage mechanism; and

FIG. 7D is a diagram illustrating another embodiment of construct for a spinal stabilization device having two pivoting joints to kinematically form a four bar linkage mechanism.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides various methods and devices for replacing damaged, injured, diseased, or otherwise unhealthy posterior elements, such as the facet joints, the lamina, the posterior ligaments, and/or other features of a patient's spinal column. In certain exemplary embodiments, a four bar linkage mechanism can be used to construct spinal stabilization devices and methods for restoring function to adjacent vertebrae. A four bar linkage mechanism is a mechanism that lies in a plane and that consists of four linkages that are connected by four joints that allow movement in the plane of the mechanism. Typically, one of the linkages is fixed so that it does not move. As applied to the spinal column, a spinal stabilization device can be constructed to dynamically form a four bar linkage mechanism, when movement is viewed in the sagittal plane, with the adjacent vertebrae and a disc (or some other element) disposed between the adjacent vertebrae. In particular, with reference to FIG. 1, one of the vertebrae, e.g., an inferior vertebra, can form a first, fixed linkage L₁. Since the disc moves relative to the inferior vertebra, the disc can form a second linkage L₂ that is coupled to the first linkage L₁ (inferior vertebra). Movement between the second linkage L₂ (disc) and the first linkage L₁ (inferior vertebra) can be represented as a first joint J₁. The disc can also move relative to an adjacent superior vertebra, and thus the superior vertebra can form a third linkage L₃. Movement between the third linkage L₃ (superior vertebra) and the second linkage L₂ (disc) can be represented as a second joint J₂. The superior vertebra can, in turn, be movably coupled to the inferior vertebra by a body, which can form a fourth linkage L₄. Movement between the fourth linkage L₄ (body) and the third linkage L₃ (superior vertebra) can be represented as a third joint J₃, and movement between the fourth linkage L₄ (body) and the first linkage L₁ (inferior vertebra) can be represented as a fourth joint J₄. Accordingly, the inferior vertebra, the disc, the superior vertebra, and a body together can kinematically form a four bar linkage mechanism in the sagittal plane. A person skilled in the art will appreciate that, while FIG. 1 illustrates a substantially rectangular four bar linkage mechanism, the length and angular orientation of the linkages L₁-L₄ can vary. A person skilled in the art will also appreciate that the spinal stabilization device can be disposed on any posterior portion of the spinal column, and that the structure of the device is not limited to be positioned in the sagittal plane. The four bar linkage mechanism is merely used to explain the resulting movement that occurs when the device is viewed in the sagittal plane.

This four bar linkage mechanism can be used to construct a variety of spinal stabilization devices and methods for restoring function to adjacent vertebrae. For example, while the body that forms the fourth linkage L₄ is preferably rigid, at least in the sagittal plane, the body can have a variety of shapes, sizes, and orientations, and it can be coupled to the adjacent vertebrae using a variety of joints J₃, J₄. The joints J₃, J₄ can be, for example, joints that pivot in the sagittal plane, joints that slide in the sagittal plane, or combinations thereof. The disc that forms the second linkage L₂, as well as the joints J₁, J₂ that allow movement of the disc relative to the adjacent vertebrae, can also have a variety of configurations. For example, the disc can be a natural disc, an artificial disc, or any other element that is disposed between the adjacent vertebrae and that allows at least two degrees of freedom when implanted between adjacent vertebrae. The joints J₁, J₂ that allow movement of the disc can also be sliding and/or pivoting joints. Accordingly, a person skilled in the art will appreciate that a variety of techniques can be used to provide spinal stabilization devices that kinematically form a four bar linkage mechanism in the sagittal plane. A person skilled in the art will also understand that the various exemplary stabilization devices described and shown herein are merely relied on for illustration purposes to demonstrate various constructs that kinematically form a four bar linkage mechanism in the sagittal plane when implanted.

At the outset, it is important to note that since a natural disc has three degrees of freedom when viewed in the sagittal plane, it normally could not be considered to dynamically form one of the rigid linkages of a four bar linkage mechanism. Regardless, a person skilled in the art will appreciate that the methods and devices disclosed herein are not intended to be limited to use with spinal discs having only two degrees of freedom, even though conceptually a disc have two degrees of freedom in the sagittal plane is necessary to form a four bar linkage concept. The methods and devices will function properly with a natural disc, and thus any reference herein to a disc that forms a linkage of a four bar linkage mechanism in the sagittal plane is intended to include a natural disc, i.e., a disc have two or more degrees of freedom.

FIGS. 2A and 2B illustrate one exemplary embodiment of a spinal stabilization device 10 that kinematically forms a four bar linkage mechanism in the sagittal plane when connected between adjacent vertebrae Vs, Vi. As shown, the device 10 generally includes a first connecting element 12 that is adapted to rigidly couple to a first vertebra, e.g., a superior vertebra Vs, and a second connecting element 14 that is adapted to rigidly couple to a second adjacent vertebra, e.g., an inferior vertebra Vi. A variety of techniques can be used to attach the connectors 12, 14 to the adjacent vertebrae Vs, Vi, but in the illustrated embodiment the connectors 12, 14 are mated to the vertebrae Vs, Vi using bone screws to form a rigid connection. The device 10 also includes first and second bodies 16, 18 that are adapted to movably couple to the first and second connectors 12, 14 to allow movement of the adjacent vertebrae Vs, Vi. While various techniques can be used to movably couple each body 16, 18 to the connectors 12, 14, in the illustrated exemplary embodiment each body 16, 18 includes a first bore 16 a, 18 a formed therein and adapted to receive the first connector 12, and a second bore 16 b, 18 b formed therein and adapted to receive the second connector 14. The first bore 16 a, 18 a in each body 16, 18 allows the first connector 12 to slide relative to the bodies 16, 18 thereby allowing flexion of the adjacent vertebrae Vs, Vi coupled thereto. The bodies 16, 18 can also optionally be flexible to allow additional flexion and/or to control movement of the adjacent vertebrae. The spinal stabilization device 10 and other exemplary embodiments of spinal stabilization devices are described in more detail in U.S. patent application Ser. No. 10/955,207, filed on Sep. 30, 2004 and entitled “Posterior Stabilization Systems and Methods.”

FIG. 2B illustrates the device 10 in the sagittal plane, showing one of the bodies, e.g., body 16 movably coupled to the first and second connectors 12, 14, which in turn are coupled to the adjacent vertebrae Vs, Vi. As is further shown, an artificial disc 100 is implanted between the adjacent vertebrae Vs, Vi. In general, the illustrated disc 100 includes a superior endplate member 102 that rigidly connects to the superior vertebra Vs, an inferior endplate member 104 that rigidly connects to the inferior vertebra Vi, and a core 106 movably disposed therebetween. The core 106 has convex superior and inferior surfaces 106 s, 106 i that sit within corresponding concave surfaces formed in the superior and inferior endplate members 102, 104, thereby allowing the core 106 to pivot with respect to the endplate members 102, 104. As previously noted, the spinal stabilization implants disclosed herein can be used with an artificial disc have virtually any configuration, or with a natural disc or any other element that allows movement between adjacent vertebrae Vs, Vi. In an exemplary embodiment, however, the disc is preferably adapted to provide at least two degrees of freedom when implanted between adjacent vertebrae. By way of non-limiting example, one exemplary artificial disc for use with the present invention is the Charité™ Artificial Disc available from DePuy Spine, Inc.

In use, referring to FIGS. 2B and 2C, the device 10 can kinematically form a four bar linkage mechanism in the sagittal plane to control movement of the adjacent vertebrae Vs, Vi. In particular, assuming one of the vertebrae Vs, Vi, e.g., the inferior vertebra Vi, is fixed, the fixed vertebra Vi can represent the first linkage L₁ of a four bar linkage mechanism. The core 106 forms the second linkage L₂, as the core 106 moves relative to the inferior vertebra Vi. Movement between the core 106 and the inferior vertebra Vi can be represented by a first joint J₁. The first joint j₁ is located at a center of rotation 106 a of a path of movement of the core 106 relative to the inferior vertebra Vi. As shown in FIG. 2B, the inferior surface 106 i of the core 106 moves relative to the fixed inferior vertebra Vi along circular path 108 a, and thus the center of rotation 106 a of the circular path 108 a forms the first joint J₁ between the core 106, i.e., the second linkage L₂, and the inferior vertebra Vi, i.e., the first linkage L₁. The first joint J₁ is represented in FIG. 2C as a pivot joint since the core 106 pivots relative to the inferior vertebra Vi in the sagittal plane.

Since the core 106 forms the second linkage L₂, the superior vertebra Vs forms the third linkage L₃, as the superior vertebra Vs moves relative to the core 106. The joint that allows movement between the core 106, i.e., the second linkage L₂, and the superior vertebra Vs, i.e., the third linkage L₃, is determined by a center of rotation of a path of movement of the core 106 relative to the superior vertebra Vs. As shown in FIG. 2B, the superior surface 106 s of the core 106 moves relative to the superior vertebra Vs along a circular path 108 b, and thus the center of rotation 106 b of the circular path 108 b forms the second joint J₂ between the core 106, i.e., the second linkage L₂, and the superior vertebra Vs, i.e., the third linkage L₃. The second joint J₂ is represented in FIG. 2C as a pivot joint since the core 106 pivots relative to the superior vertebra Vs in the sagittal plane.

As is further shown in FIGS. 2B and 2C, the body 16 forms the fourth linkage L₄, as the body 16 moves relative to the superior vertebra Vs and the inferior vertebra Vi. The joint that allows movement between the superior vertebra Vs, i.e., the third linkage L₃, and the body 16, i.e., the fourth linkage L₄, is again determined by the center of rotation of a path of movement of the body 16 relative to the superior vertebra Vs. As shown in FIG. 2B, the body 16 slides in a superior-inferior direction along a straight path. Accordingly, the third joint J₃ that couples the body 16, i.e., the fourth linkage L₄, to the superior vertebra, i.e., the third linkage L₃, is represented as a sliding joint, shown in FIG. 2C, since the body 16 slides in the sagittal plane relative to the superior vertebra Vs. The center of rotation of the path of movement of a straight line is infinite, and thus the third joint J₃ that allows movement between the third and fourth linkages L₃, L₄ is positioned an infinite distance away along the horizontally-extending line that is perpendicular to the sliding joint. In other words, the third and fourth linkages L₃, L₄ that are coupled by the third joint J₃ can each have an infinite length, as shown in FIG. 2E.

Continuing to refer to FIGS. 2B and 2C, the body 16, which forms the third linkage L₃, also moves relative to the inferior vertebra Vi, i.e., the first, fixed linkage L₁. The fourth joint J₄ of the four bar linkage mechanism that allows movement of the body 16 relative to the inferior vertebra Vi is located at a center of rotation of the path of movement of the body 16 relative to the inferior vertebra Vi. As shown in FIG. 2B, the center of rotation is the bore 16 b formed in the body 16, as the body 16 pivots about the connector 14 that extends through the bore 16 b and that is rigidly coupled to the inferior vertebra Vi. Since the body 16 pivots relatives to the inferior vertebra Vi, the fourth joint J₄ is represented as a pivot joint in FIG. 2C. As previously indicated, the device 10 can include two bodies 16, 18, and thus body 18 can provide motion similar to that provided by body 16. In other words, the device 10 can include any number of bodies aligned in the frontal plane to provide movement between the adjacent vertebrae.

In sum, the spinal stabilization device 10 kinematically forms a four bar linkage mechanism in the sagittal plane with adjacent superior and inferior vertebrae Vs, Vi coupled thereto, and with a disc 100 disposed between the adjacent superior and inferior vertebrae Vs, Vi. The four bar linkage mechanism is illustrated in FIGS. 2D and 2E, and it includes four linkages L₁-L₄ coupled by three pivoting joints J₁, J₂, and J₄ and a sliding joint J₃.

FIG. 3A illustrates another exemplary embodiment of a spinal stabilization device 20 that kinematically forms a four bar linkage mechanism in the sagittal plane when connected between adjacent vertebrae Vs, Vi. As shown, the device 20 generally includes a first connecting element 22 that is adapted to rigidly couple to a first vertebra, e.g., a superior vertebra Vs, and a second connecting element 24 that is adapted to rigidly couple to a second adjacent vertebra, e.g., an inferior vertebra Vi. The first and second connecting elements 22, 24 can each have a variety of shapes and sizes, but in the illustrated embodiment the first connecting element 22 is in the form of an extension rod having a first portion that is adapted to rigidly mate to the superior vertebra Vs, and a second portion that is adapted to slidably couple to a body 26, and the second connecting element 24 is in the form of an L-shaped member having a first portion that is adapted to rigidly mate to the inferior vertebra Vi, and a second portion that is adapted to pivotally couple to a body 26. A variety of techniques can be used to attach the connectors 22, 24 to the adjacent vertebrae Vs, Vi, but in the illustrated embodiment the connectors 22, 24 are mated to the vertebrae Vs, Vi using bone screws to form a rigid connection.

The device 20 also includes a body 26 that is adapted to movably couple to the first and second connectors 22, 24 to allow movement of the adjacent vertebrae Vs, Vi. While various techniques can be used to movably couple the body 26 to the connectors 22, 24, in the illustrated exemplary embodiment the body 26 is in the form of a ball bearing that is rotatably disposed within a socket 24 a formed in the second connector 24, and that has a bore 26 a formed therethrough for slidably receiving the first connector 22. In use, the sliding joint allows the first connector 22 to slide relative to the body 26, and the ball and socket joint allows the body 26 to pivot relative to the second connector 24, thereby allowing flexion of the adjacent vertebrae Vs, Vi coupled thereto. The spinal stabilization device 20 and other exemplary embodiments of spinal stabilization devices are described in more detail in U.S. patent application Ser. No. 10/905,374, filed on Dec. 30, 2004 and entitled “Artificial Facet Joints,” and in U.S. patent application Ser. No. 10/908,882, filed May 31, 2005 and entitled “Facet Joint Replacement.”

As is further shown in FIG. 3A, the spinal stabilization device 20 can also be used with artificial disc 100, previously described with respect to FIG. 2A. A person skilled in the art will appreciate that, while artificial disc 100 is shown, the spinal stabilization implants disclosed herein can be used with an artificial disc have virtually any configuration, or with a natural disc or any other element that allows movement between adjacent vertebrae Vs, Vi. However, as previously discussed, the disc is preferably adapted to provide at least two degrees of freedom when implanted between adjacent vertebrae.

In use, referring to FIG. 3B, the device 20 can kinematically form a four bar linkage mechanism in the sagittal plane to control movement of the adjacent vertebrae Vs, Vi. As previously explained with respect to FIGS. 2B and 2C, assuming the inferior vertebra Vi is fixed and represents the first linkage L₁ of a four bar linkage mechanism, the core 106 of the disc 100 forms the second linkage L₂ that is coupled to the inferior vertebra Vi by a first joint J₁, which is located at the center of rotation 106 a of a circular path of movement 108 a of the inferior surface 106 i of the core 106. Likewise, as previously explained, the superior vertebra Vs forms the third linkage L₃ that is movably coupled to the second linkage L₂, e.g., core 106, by a second joint J₂, which is located at the center of rotation 106 b of a circular path of movement 108 b of the superior surface 106 s of the core 106. The first and second joints J₁, J₂ are each represented in FIG. 3B as pivot joints since the core 106 pivots relative to the superior and inferior vertebrae Vs, Vi in the sagittal plane.

As is further shown in FIGS. 3A and 3B, the body 26 forms the fourth linkage L₄, as the body 26 moves relative to the superior vertebra Vs and the inferior vertebra Vi. The joint that allows movement between the superior vertebra Vs, i.e., the third linkage L₃, and the body 26, i.e., the fourth linkage L₄, is again determined by the center of rotation of a path of movement of the body 26 relative to the superior vertebra Vs. As shown in FIG. 3A, the body 26 is slidably movable relative to the first connector 22, which is rigidly coupled to the superior vertebra Vs, and the path of movement of the body 26 extends in a superior-inferior direction along a straight path. Accordingly, the third joint J₃ that couples the body 26, i.e., the fourth linkage L₄, to the superior vertebra, i.e., the third linkage L₃, is represented as a sliding joint, shown in FIG. 3B, since the body 26 slides in the sagittal plane relative to the superior vertebra Vs. As previously explained, the center of rotation of the path of movement of a straight line is infinite, and thus the third joint J₃ that allows movement between the third and fourth linkages L₃, L₄ is positioned an infinite distance away along the horizontally-extending line that is perpendicular to the sliding joint. In other words, the third and fourth linkages L₃, L₄ that are coupled by the third joint J₃ can each have an infinite length, as previously shown in FIG. 2E.

Continuing to refer to FIGS. 3A and 3B, the body 26, which forms the third linkage L₃, also moves relative to the inferior vertebra Vi, i.e., the first, fixed linkage L₁. In particular, the body 26 pivots relative to the second connector 26 that is rigidly attached to the inferior vertebra Vi. The fourth joint J₄ of the four bar linkage mechanism that allows movement of the body 26 relative to the inferior vertebra Vi is therefore a pivoting joint that is located at a center of rotation of the path of movement of the body 26 relative to the inferior vertebra Vi. In the illustrated embodiment, the center of rotation is the bore 26 a formed in the body second connector 26, as the body 26 pivots within the bore 26 a.

In sum, the spinal stabilization device 20 kinematically forms a four bar linkage mechanism in the sagittal plane with adjacent superior and inferior vertebrae Vs, Vi coupled thereto, and with a disc 100 disposed between the adjacent superior and inferior vertebrae Vs, Vi. The four bar linkage mechanism is illustrated in FIG. 3C, and it includes four linkages L₁-L₄ coupled by three pivoting joints J₁, J₂, and J₄ and a sliding joint J₃.

FIGS. 4A and 4B illustrate another exemplary embodiment of a spinal stabilization device 30 that kinematically forms a four bar linkage mechanism in the sagittal plane when connected between adjacent vertebrae Vs, Vi. As shown, the device 30 generally includes a first connecting element 32 that is adapted to rigidly couple to a first vertebra, e.g., a superior vertebra Vs, and a second connecting element 34 that is adapted to rigidly couple to a second adjacent vertebra, e.g., an inferior vertebra Vi. The first and second connecting elements 32, 34 can each have a variety of shapes and sizes, but in the illustrated embodiments the first and second connecting elements 32, 34 each have an elongate central portion that is adapted to couple to a body 36, and opposed arms extending from the elongate central portion for mating to the vertebrae Vs, Vi. A variety of techniques can be used to attach the connectors 32, 34 to the adjacent vertebrae Vs, Vi, but in the illustrated embodiment the connectors 32, 34 are mated to the vertebrae Vs, Vi using bone screws to form a rigid connection.

The device 30 also includes a body 36 that is adapted to movably couple to the first and second connectors 32, 34 to allow movement of the adjacent vertebrae Vs, Vi. While various techniques can be used to movably couple the body 36 to the connectors 32, 34, in the illustrated exemplary embodiment the body 36 is in the form of a triangular member that rotatably mates to a bar 32 a formed on the first connecting element 32, and that slides within a pathway 34 a formed in the second connecting element 34. As a result of the sliding and pivoting joints, the body 36 allows the first and second connectors 32, 34 to move relative to one another, thereby allowing flexion of the adjacent vertebrae Vs, Vi coupled thereto. The spinal stabilization device 30 and other exemplary embodiments of spinal stabilization devices are described in more detail in U.S. patent application Ser. No. 10/905,376, filed on Dec. 30, 2004 and entitled “Posterior Stabilization System.”

As is further shown in FIGS. 4A and 4B, the spinal stabilization device 30 can also be used with artificial disc 100, previously described with respect to FIG. 2A. A person skilled in the art will appreciate that, while artificial disc 100 is shown, the spinal stabilization implants disclosed herein can be used with an artificial disc have virtually any configuration, or with a natural disc or any other element that allows movement between adjacent vertebrae Vs, Vi. Again, in an exemplary embodiment, the disc is preferably adapted to provide at least two degrees of freedom when implanted between adjacent vertebrae.

In use, referring to FIGS. 4B and 4C, the device 30 can kinematically form a four bar linkage mechanism in the sagittal plane to control movement of the adjacent vertebrae Vs, Vi. As previously explained with respect to FIGS. 2B and 2C, assuming the inferior vertebra Vi is fixed and represents the first linkage L₁ of a four bar linkage mechanism, the core 106 of the disc 100 forms the second linkage L₂ that is coupled to the inferior vertebra Vi by a first joint J₁, which is located at the center of rotation 106 a of a circular path of movement 108 a of the inferior surface 106 i of the core 106. Likewise, as previously explained, the superior vertebra Vs forms the third linkage L₃ that is movably coupled to the second linkage L₂, e.g., core 106, by a second joint J₂, which is located at the center of rotation 106 b of a circular path of movement 108 b of the superior surface 106 s of the core 106. The first and second joints J₁, J₂ are each represented in FIG. 4C as pivot joints since the core 106 pivots relative to the superior and inferior vertebrae Vs, Vi in the sagittal plane.

As is further shown in FIGS. 4B and 4C, the body 36 forms the fourth linkage L₄, as the body 36 moves relative to the superior vertebra Vs and the inferior vertebra Vi. The joint that allows movement between the superior vertebra Vs, i.e., the third linkage L₃, and the body 36, i.e., the fourth linkage L₄, is again determined by the center of rotation of a path of movement of the body 36 relative to the superior vertebra Vs. In this embodiment, as shown in FIG. 4B, the body 36 pivots relative to the first connector 34 that is rigidly attached to the superior vertebra Vs. The third joint J₃ of the four bar linkage mechanism that allows movement of the body 36 relative to the superior vertebra Vs is therefore a pivoting joint that is located at a center of rotation of the path of movement of the body 36 relative to the superior vertebra Vi. In the illustrated embodiment, the center of rotation is the bar 32 a formed on first connector 32, as the body 36 pivots about the bar 32 a.

Continuing to refer to FIGS. 4B and 4C, the body 36, which forms the third linkage L₃, also moves relative to the inferior vertebra Vi, i.e., the first, fixed linkage L₁. In particular, the body 36 slidably moves along a circular path 34 b relative to the second connector 34, which is rigidly coupled to the inferior vertebra Vi. The center of rotation 34 c of the circular path 34 b thus forms the fourth joint J₄ that couples the body 36, i.e., the fourth linkage L₄, to the inferior vertebra Vi, i.e., the first linkage L₁. The fourth joint J₄ is represented as a pivoting joint, shown in FIG. 4C, since the body 36 slidably moves about a circular path in the sagittal plane relative to the inferior vertebra Vi. A person skilled in the art will appreciate that the joints J₃ and J₄ can be positioned at different locations. For example, although joint J₄ currently lies near the center of the disc, the joint J₄ can be placed more posteriorly, forming a shorter link L₄. The joint J₄ could also be placed posterior to joint J₃.

In sum, the spinal stabilization device 30 kinematically forms a four bar linkage mechanism in the sagittal plane with adjacent superior and inferior vertebrae Vs, Vi coupled thereto, and with a disc 100 disposed between the adjacent superior and inferior vertebrae Vs, Vi. The four bar linkage mechanism is illustrated in FIG. 4D, and it includes four linkages L₁-L₄ coupled by four pivoting joints J₁-J₄.

In each of the various embodiments described above, the spinal stabilization devices 10, 20, 30 each kinematically form a four bar linkage mechanism in the sagittal plane with adjacent superior and inferior vertebrae Vs, Vi and a core 106 disposed between the vertebrae Vs, Vi. As previously noted, the four bar linkage mechanism can be used to construct a variety of other spinal stabilization devices for restoring function to adjacent vertebrae. A portion of the four bar linkage mechanism can have a construct as shown in FIGS. 5A and 5B, wherein the inferior vertebra Vi forms the first, fixed linkage L₁, the core 106 (or any other disc or element disposed between the vertebrae Vs, Vi) forms the second linkage L₂ that is coupled to the first linkage L₁ by a first pivot joint J₁ located at a center of rotation of the path of movement 108 a of the inferior surface 106 i of the core 106 relative to the inferior vertebra Vi, and the superior vertebra Vs forms the third linkage L₃ that is coupled to the second linkage L₂ by a second pivot joint J₂ located at a center of rotation of the path of movement 108 b of the superior surface 106 s of the core 106 relative to the superior vertebra Vs. The remainder of the four bar linkage mechanism can be constructed to provide a fourth linkage L₄, e.g., a body, and third and fourth joints J₃, J₄, each of which can have virtually any configuration. As previously indicated, the joints J₃, J₄ can be sliding joints such as a sleeve and rod, pivoting joints such as a ball and socket, or some combination thereof. In certain exemplary embodiments, the third and fourth joints J₃, J₄ are two pivoting joints, or one sliding joint and one pivoting joint. Where a sliding joint is used, the joint can be a straight sliding joint or a curved sliding joint. Where a straight sliding joint is used, sliding movement can occur in a generally vertical direction (i.e., in a superior-inferior direction). Where a curved sliding joint is used, the linkage L₄ that connects to the joints J₃, J₄ extends in generally horizontal direction, as previously described. In other words, the center or rotation of each joint J₃, J₄, where one joint is a curved sliding joint, is aligned generally horizontally.

By way of non-limiting example, FIGS. 6A-7D illustrate a variety of joint combinations and orientations for forming a spinal stabilization device that kinematically forms a four bar linkage mechanism in the sagittal plane with adjacent vertebrae and a disc disposed therebetween. In FIGS. 6A-6H, each four bar linkage mechanism includes one sliding joint and one pivoting joint. The orientation of each joint is varied to illustrate some possible configurations for forming a spinal stabilization device. For example, in FIG. 6A the four bar linkage mechanism includes a first, fixed linkage L₁ (first vertebra), a second linkage L₂ (disc) that is pivotally coupled to the first linkage L₁ by a first joint J₁, a third linkage L₃ (second adjacent vertebra) that is pivotally coupled to the second linkage L₂, and a fourth linkage L₄ (body) that is slidably coupled to the third linkage L₃ and that is pivotally coupled to the first linkage L₁. In this embodiment, the fourth linkage L₄ (body) includes a rod that is slidably disposed through a sleeve formed on the third linkage L₃ to form the third joint J₃, and a ball that is pivotally disposed within a socket formed on the first linkage L₁ to form the fourth joint J₄. While the first and third linkages L₁, L₃ are representative of first and second adjacent vertebrae, the linkages are also representative of any components that are rigidly coupled to the adjacent vertebrae. Thus, the sleeve of the third linkage L₃ can be formed on a connecting element that is rigidly mated to the second vertebra, and the socket of the first linkage L₁ can be formed in a connecting element that is rigidly mated to the first vertebra.

FIGS. 6B and 6C illustrate other embodiments of a four bar linkage mechanism that are similar to the four bar linkage mechanism shown in FIG. 6A. However, in the embodiment shown in FIG. 6B the fourth linkage L₄ (body) includes a socket formed thereon that rotatably seats a ball formed on the third linkage L₃ (second vertebra) to form the third joint J₃, and a rod formed thereon that is slidably disposed through a sleeve formed on the first linkage (first vertebra) to form the fourth joint J₄. In the embodiment shown in FIG. 6C, the fourth linkage L₄ (body) includes a ball formed thereon that is rotatably disposed within a socket formed in the third linkage L₃ (second vertebra) to form the third joint J₃, and a rod formed thereon that is slidably disposed through a sleeve formed on the first linkage L₁ (first vertebra) to form the fourth joint J₄.

FIG. 6D illustrates another possible configuration in which the fourth linkage L₄ (body) is in the form of a ball bearing that forms both the third and fourth joints J₃, J₄. In particular, the ball bearing includes a bore extending therethrough that forms a sleeve for slidably receiving the third linkage L₃ (second vertebra) to form the third joint J₃, and it is rotatably disposed within a socket formed in the first linkage L₁ (first vertebra) to form the fourth joint J₄. FIGS. 6E-6H illustrate other various combinations of a fourth linkage L₄ having a sliding joint and a pivoting joint.

FIGS. 7A-7D likewise illustrate various combinations for forming a four-bar linkage mechanism, however in these embodiments the third and fourth joints J₃, J₄ that couple the fourth linkage L₄ to the third and first linkages L₃, L₁ are pivoting joints, e.g., ball and socket joints. In FIG. 7A, the fourth linkage L₄ (body) includes a first socket formed therein for receiving a ball formed on the third linkage L₃ (second vertebra) to form the third joint J₃, and a second socket formed thereon for receiving a ball formed on the fourth linkage L₄ (first vertebra) to form the fourth joint J₄. In FIG. 7B, the fourth linkage L₄ (body) includes a first socket formed therein for receiving a ball formed on the third linkage L₃ (second vertebra) to form the third joint J₃, and a ball formed thereof that is rotatably disposed within a socket formed on the fourth linkage L₄ (first vertebra) to form the fourth joint J₄. FIGS. 7C and 7D likewise illustrate various other combinations for forming two ball and socket joints to couple the fourth linkage L₄ to the third and first linkages L₃, L₁. A person skilled in the art will appreciate that FIGS. 6A-7D merely illustrate some possible combinations for coupling a fourth linkage L₄ (body) of a four bar linkage mechanism to the third and first linkages L₃, L₁, and that a variety of other configurations are possible and can be used to construct a spinal stabilization device that kinematically forms a four bar linkage mechanism in the sagittal plane with adjacent vertebrae and a disc disposed therebetween.

A person skilled in the art will appreciate that, while the sliding joints and pivoting joints shown in FIGS. 6A-6H and 7A-7D each represent one translational or rotational degree-of-freedom, that multiple pieces may be used to construct each joint. For example, a sliding joint can be composed of two simple mating surfaces, such as a sleeve and a rod, or it can be composed of multiple pieces, such as a linear ball bearing having multiple balls housed in a casing, to accommodate a sliding motion. A pivoting joint can also be composed of two simple mating surfaces, such as a ball and socket, or it can be composed of multiple pieces, such as a hinge joint with a cylindrical rod.

One skilled in the art will appreciate further features and advantages of the invention based on the above-described embodiments. Accordingly, the invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety. 

1. An implant for stabilizing adjacent vertebrae, comprising: a rigid body having a first joint for movably coupling to a first vertebra and a second joint for movably coupling to a second adjacent vertebra such that the rigid body kinematically forms a four bar linkage mechanism in a sagittal plane with the first and second adjacent vertebrae and a disc disposed between the first and second adjacent vertebrae for restoring function to the first and second adjacent vertebrae, the first and second joints each having one degree of freedom in the sagittal plane.
 2. The implant of claim 1, wherein the first joint is a sliding joint and the second joint is a rotating joint, the sliding joint and the rotating joint together being adapted to allow flexion and extension of first and second adjacent vertebrae.
 3. The implant of claim 2, wherein the sliding joint is adapted to slide along a curved path in the sagittal plane.
 4. The implant of claim 2, wherein the sliding joint comprises a sleeve adapted to slidably receive a rod.
 5. The implant of claim 2, wherein the rotating joint comprises a ball adapted to be rotatably disposed within a socket.
 6. The implant of claim 2, wherein the rotating joint is a hinge joint.
 7. The implant of claim 1, wherein the first and second joints each comprise sliding joints that are adapted to allow flexion and extension of first and second adjacent vertebrae.
 8. The implant of claim 7, wherein the sliding joints each comprise a sleeve that is adapted to slidably receive a rod.
 9. The implant of claim 1, wherein the first and second joints are each rotating joints that are adapted to allow flexion and extension of first and second adjacent vertebrae.
 10. The implant of claim 1, wherein a center of rotation of each of the first and second joints is positioned substantially horizontal relative to one another.
 11. The implant of claim 1, wherein at least one of the first and second joints is adapted to slide vertically when the first and second joints are coupled to adjacent vertebrae.
 12. An implant for stabilizing adjacent vertebrae, comprising: a rigid body having a first joint for movably coupling to a first vertebra and a second joint for movably coupling to a second adjacent vertebra, the first and second joints each being adapted to provide one degree of freedom in a sagittal plane when coupled to adjacent vertebrae such that the rigid body is adapted to restore function to adjacent vertebrae coupled thereto in combination with a disc disposed between the adjacent vertebrae.
 13. The implant of claim 12, wherein a center of rotation of each of the first and second joints is positioned substantially horizontal relative to one another.
 14. The implant of claim 12, wherein at least one of the first and second joints is adapted to slide vertically when the first and second joints are coupled to adjacent vertebrae.
 15. An implant for stabilizing adjacent vertebrae in a patient's spine, comprising: a first linkage adapted to rigidly couple to a first vertebra; a second linkage adapted to rigidly couple to a second adjacent vertebra; and a rigid body having a first joint for movably coupling to the first linkage, and a second joint for movably coupling to the second linkage, the first and second joints each having one degree of freedom in a sagittal plane when implanted; wherein the first and second linkages and the body are adapted to kinematically form a four bar linkage mechanism in a sagittal plane when implanted with a disc disposed between first and second adjacent vertebrae to restore function to the first and second adjacent vertebrae.
 16. The implant of claim 15, wherein the first and second joints are each adapted to slidably move relative to the first and second linkages.
 17. The implant of claim 16, wherein the first and second joints each comprise a sleeve that is adapted to slidably receive a rod formed on the first and second linkages.
 18. The implant of claim 15, wherein the first joint is adapted to slidably move relative to the first linkage, and the second joint is adapted to rotatably move relative to the second linkage.
 19. The implant of claim 18, wherein the first joint comprises a sleeve that is adapted to slidably receive a rod formed on the first linkage, and the second joint comprises a ball that is adapted to be rotatably disposed within a socket formed in the second linkage.
 20. The implant of claim 19, wherein the sleeve comprises a lumen formed in the ball.
 21. A method for restoring function to adjacent superior and inferior vertebrae, comprising: coupling adjacent superior and inferior vertebrae with a moving linkage to kinematically form a four bar linkage mechanism in a sagittal plane with the adjacent superior and inferior vertebrae and a disc disposed between the adjacent superior and inferior vertebrae, the moving linkage having first and second joints each with one degree of freedom in the sagittal plane.
 22. The method of claim 21, wherein the disc comprises an artificial disc adapted to allow movement between the adjacent superior and inferior vertebrae.
 23. The method of claim 21, wherein one of the joints of the moving linkage is a sliding joint, and the other one of the joints of the moving linkage is a pivoting joint.
 24. The method of claim 21, wherein the first and second joints of the moving linkage are each sliding joints.
 25. The method of claim 21, wherein coupling adjacent superior and inferior vertebrae with a moving linkage comprises rigidly mating a first member to a superior vertebra, rigidly mating a second member to an inferior vertebra, and coupling the first and second joints of the moving linkage to the first and second members.
 26. The method of claim 25, wherein one of the first and second joints allows the moving linkage to slide relative to one of the first and second members, and the other one of the first and second joints allows the moving linkage to pivot relative to the other one of the first and second members.
 27. The method of claim 25, wherein the first and second joints allow the moving linkage to slide relative to each of the first and second members.
 28. The method of claim 25, wherein coupling adjacent superior and inferior vertebrae with a moving linkage further comprises implanting an artificial disc between the adjacent superior and inferior vertebrae. 